CHIANG, Y.-M.; GAO, S.-A. On a problem in complex oscillation theory of periodic second order linear differential equations and some related perturbation results. Annales Fennici Mathematici, [S. l.], v. 27, n. 2, p. 273–290, 2002. Disponível em: https://afm.journal.fi/article/view/135043. Acesso em: 18 may. 2024.